I need to turn a 490Hz PWM into a DC voltage using a Sallen Key low pass filter.
In the picture, you can see my bode plot and there is -60dB attenuation at 490Hz. The passband has 0dB of gain. The load is the 10k resistor. (Don't ask about the weird capacitor values it's what I got.)
The results from the simulator are perfect. For a 50% duty cycle I got an output voltage of 2.5V.
When I went to build this circuit here's the output I actually am getting:
Now I'm fairly certain that my circuit is working somewhat properly, because here is the signal at the positive terminal of the op-amp.It is a DC voltage of about 2.4V with a little ripple.
Here's what I've done so far to fix the problem:
- Switched out the op amp for another one to see if it was just broken.
- Checked to make sure that all of my wiring is correct (it is I've checked 100 times.)
- Verified component values.
- I've checked the signal at every "node" and compared it with my simulation. The signal at every single node matches up with simulation except for the output pin.
It feels like maybe the DC part of my + input signal is getting rejected and the ripple is the only thing getting through.